That's because Mount Ontake is a volcano, and this volcano decided at that moment to erupt, as reported by the BBC. The eruption trapped 150 climbers, including those with Terutoshi, who just happened to catch the whole thing on video.
According to Reuters, at least one person has died and over 30 have been seriously injured since the volcano began raining ash on central Japan this morning. Prime Minister Shinzo Abe ordered the military to rescue the remaining hikers, telling reporters:
"I instructed to do all we can to rescue the people affected and secure the safety of the trekkers."
Check it out, it is terrifying: